1. Field of the Invention
The present invention relates to prisms, and more particularly to a color-separating prism for use in an electronic imaging system such as a digital camera.
2. Background of the Invention
Electronic color imaging devices, such as video cameras or digital still cameras, typically record color images by independently sensing the scene to be recorded in red (long wavelength), green (medium wavelength) and blue (short wavelength) color channels. Such devices conventionally employ a prism to separate an incoming light beam into red, green, and blue color components, and to direct these components onto corresponding sensors. Signals generated by the sensors are then stored and subsequently utilized to reproduce the recorded scene.
In order to achieve colorimetrically accurate imaging in imaging devices of the foregoing description, the sensitivity curves associated with the color channels must closely approximate a set of color matching functions. A departure from this condition may result in color reproduction errors wherein colors that appear alike to a viewer's eye are reproduced differently, or colors which appear differently to a viewer's eye are reproduced as identical colors. Color reproduction errors may be particularly undesirable in certain types of print media. For example, a photograph of an article of clothing printed in a catalog may depict the clothing as having a certain hue which departs significantly from actuality, thereby leading to dissatisfied customers and returned merchandise. Color reproduction errors may be reduced by making post-imaging adjustments with color correction equipment; however, such equipment is expensive, and its operation is time-consuming and complex.
Numerous references disclosing color-separating prism designs appear in the prior art. These references have uniformly taught away from attempting to approximate the relatively complex, multi-peaked spectral distributions represented by color matching functions, and have instead taught the use of dichroic or filter elements which separate the incoming light into components individually comprising sharply defined bands or ranges of wavelengths. For example, U.S. Pat. No. 3,659,918 to Tan teaches a prism in which "light within a given wavelength range [is] reflected at each respective layer." Similarly, U.S. Pat. No. 4,268,119 to Hartmann teaches a color separation system having a sharp cutoff between the color components, with the transmittance curve for each component rising from 10% to 90% of peak transmittance over a spectral range of only about 20 nm. Applicant has found that color separation prisms used in commercially available imaging systems typically exhibit the sharply-defined flat-topped spectral response taught by the aforementioned patent prior art. Consequently, the spectral sensitivities of these imaging systems depart substantially from color matching functions, thereby causing color reproduction errors.
In view of the foregoing discussion, there is a need in the color imaging art for a color separating prism which generates red, blue and green color channels having spectral sensitivities approximating a set of color matching functions. Utilization of a prism of this description will enable highly colorimetrically accurate color imaging and thereby minimize or eliminate color reproduction errors.